Phytoplasma
Phytoplasma:-
Introduction of Mycoplasma:-
> Mycoplasmas are the smallest among the known aerobic prokaryotes.
> They were first discovered by Pasteur in 1843, during his work on the possible causal agent of pleuropneumonia of cattle. Thus they were called pleuro- pneumonia-like organism (PPLO). Pasteur was unable to isolate them in pure culture.
> Later, Nocard and Roux (1898), the French microbiologists, were successful in growing them in pure culture-medium containing serum and confirmed by inoculation and subsequent expression of disease in healthy cattle.
> Mycoplasmas are commonly found in soil, hot spring, sewage water and also in plants and animals including man. Borrel (1910) named these organisms Asterococcus mycoides. Later, in 1929, Nowak placed them under the genus Mycoplasma.
General Characteristics of Mycoplasma:-
> They are unicellular, smallest, non-motile and prokaryotic organisms forming fried egg shaped colonies.
> They are pleomorphic i.e., able to change their shape depending upon culture media.
> They may be rod like, ring like, globoid or filamentous. The filaments are of uniform diameter (100-300 nm) and vary in length from 3 nm to 150 nm.
> Some mycoplasma predominantly assume spherical shape (300-800 nm in diameter).
> They are ultra-filterable i.e., they can pass through bacteria-proof filters.
> They do not possess rigid cell wall.
> The cells are delimited by soft tripple layered lipo-proteinaceous membrane. It is unit membrane about 10 nm thick.
> Within the cytoplasm ribosomes are found scattered in the peripheral zone. These are 14 nm in diameter and resemble with bacteria in sedimentation characteristic of both the nucleoprotein and nucleic acid.
> The ribosomes are 72S type.
> Within the cytoplasm fine fibrillar DNA is present. It is double stranded helix.
> Mycoplasma generally grow more slowly than bacteria.
> They require sterol for their nutrition.
> They are usually resistant to antibiotics like penicillin, cephaloridine, vencomycin etc. which action cell wall.
> They are sensitive to tetracycline.
> They are also killed by temperature of 40-55°C in 15 minutes.
> They do not produce spores.
> Like other prokaryotes, they usually divide by binary fission.
Structure of Mycoplasma:-
> The cell is devoid of cell wall which makes them readily deformable showing irregular and variable shapes.
> They may be ring-like, granular, coccoid, pear-shaped, filamentous, etc.
> The filaments are of two types: unbranched or branched.
> The cells are very small and measure 0.3-0.9 µm in diameter.
> The cells are covered by cytoplasmic (lipoprotein) membrane.
> Cell membrane covers the cytoplasm which contains nucleoplasm like structure and ribosomes.
> The genetic material is composed of DNA and RNA. It is about less than 50%, the amount present in other prokaryotic organisms. The amount of RNA (8%) is more than DNA (4%).
> They are usually non-motile, but some forms show gliding movements.
> They reproduce by vegetative means i.e., by binary fission and budding.
> They are sensitive to antibiotics like chloramphenicol, streptomycin, erythromycin etc., but are insensitive to penicillin, ampicillin etc., due to the absence of cell wall.
